The relationship between vision and reading achievement is complex. In this paper, a number of terms relating to vision are defined and some of the limitations of specific measures of vision are discussed. In order to relate vision to reading, it is necessary to segment arbitrarily the continuous process of vision into a series of subsystems, or functions, to allow for quantitative and qualitative discussion. The more important of these subsystems or visual skills are visual acuity, fixation, accommodation, binocular fusion, convergence, stereopsis, field of vision, and form perception. Each of these basic skills is discussed individually. A brief discussion of the interrelationships is provided. (TO)